An Introduction to ASKAP Bringing Radio Interferometers Into the Multi-pixel Era

Transcription

1 An Introduction to ASKAP Bringing Radio Interferometers Into the Multi-pixel Era Aidan Hotan and Lisa Harvey-Smith 3 rd October 2014 CSIRO ASTRONOMY AND SPACE SCIENCE

2 Introducing ASKAP The Australian SKA Pathfinder is a radio telescope array that uses new receiver technology to improve field of view and provide unprecedented survey speed. Covers a section of the radio spectrum surrounding rest-frame neutral Hydrogen emission (700 MHz to 1.8 GHz). Consists of 36 individual 12m antennas fitted with PAF receivers. Currently under construction at MRO, a radio quiet environment. Begins early science operations with a subset of antennas in Some science is already being done with a prototype 6-antenna array (BETA). It will be Australia s third major national radio astronomy facility. Total collecting area is similar to Parkes or the Compact Array Radio Astronomy School Introduction to ASKAP Aidan Hotan

3 ASKAP Science Priorities Most of the observing time will be spent on major survey projects. ASKAP will cater for large international science teams. Observations will be highly automated and done by facility operators. 10 major projects were selected to receive time during this first 5 yrs: Evolutionary Map of the Universe (EMU) Widefield ASKAP L-Band Legacy All-Sky Blind Survey (WALLABY) The First Large Absorption Survey in HI (FLASH) An ASKAP Survey for Variables and Slow Transients (VAST) The Galactic ASKAP Spectral Line Survey (GASKAP) Polarization Sky Survey of the Universe's Magnetism (POSSUM) The Commensal Real-time ASKAP Fast Transients survey (CRAFT) Deep Investigations of Neutral Gas Origins (DINGO) The High Resolution Components of ASKAP (VLBI) Compact Objects with ASKAP: Surveys and Timing (COAST) Radio Astronomy School Introduction to ASKAP Aidan Hotan

4 ASKAP Science Priorities Understanding galaxy formation and gas evolution in the nearby universe through extragalactic surveys of neutral hydrogen. Determining the evolution, formation and population of galaxies across cosmic time through radio continuum surveys. Characterise the radio transient sky through detection and monitoring of transient and variable sources. Exploring evolution of magnetic fields in galaxies over cosmic time Radio Astronomy School Introduction to ASKAP Aidan Hotan

5 Contribution to Astrophysical Knowledge Spectral Line: Survey two-thirds of the sky to z = 0.25 in neutral hydrogen. Detect 600,000 galaxies in neutral Hydrogen in 1 year. Derive distance, gas mass, total mass, dark matter content, star-forming rates and identify likely optical/ir counterparts. Continuum: Survey two-thirds of sky to RMS 10 μjy. Detect 70 million galaxies, probe evolution of star forming galaxies z < 2. Deep Field: 30 square degree field to RMS = 1μJy. Detect 0.5 million galaxies, probe evolution of star-forming galaxies z < Radio Astronomy School Introduction to ASKAP Aidan Hotan

6 ASKAP Timeline Equipment is being deployed in stages. The first 6 antennas were given Mk I prototype hardware. This sub-array is called BETA. BETA is primarily an engineering test and will be used to develop techniques. The remaining antennas will be fitted out with Mk II electronics. New hardware is being constructed now, based on lessons learned from BETA Three BETA antennas operational. First images formed BETA fully deployed, with 6 PAFs, beamformers and correlator BETA does some very early science. Initial deployment of Mk II hardware to antenna Beginning of early science with Mk II hardware on 12 antennas Radio Astronomy School Introduction to ASKAP Aidan Hotan

7 Overview of ASKAP Systems The Murchison Radio Observatory. ASKAP 3-axis antennas and array layout. Phased Array Feeds (see earlier talk). Data flow from antenna to correlator. Data flow from correlator to science data archive. Summary of results from BETA Radio Astronomy School Introduction to ASKAP Aidan Hotan

8 Murchison Radio Observatory Radio telescopes are highly susceptible to interference from signals of human origin. Astronomers call this Radio Frequency Interference (RFI). Similar to the problem of light pollution for optical observatories. Current radio observatories are in rural NSW, but civilisation has expanded to the point where some bands are unusable. The MRO is a part of Boolardy Station in Western Australia that has been set aside as a special radio quiet zone. It is one of the least-populated areas in Australia (and the world). Now has federal government protection over the use of radio transmitters within a large radius of the core site Radio Astronomy School Introduction to ASKAP Aidan Hotan

9 The Murchison Radio Observatory (MRO) 9 ASKAP Commissioning Update Aidan Hotan

10 Radio Astronomy School Introduction to ASKAP Aidan Hotan Photo taken by Alex Cherney

11 Murchison Radio Observatory Isolation is a good defence, but we must still avoid generating our own radio noise! All systems installed at the MRO must comply with stringent requirements, and will be housed in shielded structures. The only windows! Two layers of 80 db RF shielding, only optical fibre goes in or out Radio Astronomy School Introduction to ASKAP Aidan Hotan

12 ASKAP Antennas 12m parabolic reflectors, prime focus. ASKAP antennas were designed with a unique feature the ability to rotate the reflector about the optical axis. This is necessary, because Altitude- Azimuth telescopes do not naturally track the orientation of a source on the sky, only its position. Some existing telescopes allow feed rotation, but full reflector rotation better preserves the pattern of all beams on the sky Radio Astronomy School Introduction to ASKAP Aidan Hotan

13 The Value of the Roll Axis For offset beams, the roll axis allows the use of fixed weights for extended observations as a source moves across the sky. Even for the boresight beam, it improves image fidelity. See below: Image of a single beam containing 4 bright sources. Phase-only self calibration, with and without the roll axis tracking. Other parameters fixed. Image made by Ian Heywood Radio Astronomy School Introduction to ASKAP Aidan Hotan

14 PAF Assembly The ASKAP PAF is a high-tech composite structure. BETA versions were difficult to maintain and labour intensive to build. The final version is much more modular amplifiers encased in dominoes. 14 ASKAP Commissioning Update Aidan Hotan

15 Antennas and PAF Installation Radio Astronomy School Introduction to ASKAP Aidan Hotan

16 ASKAP Array Configuration 36 x 12m Antennas 16 ASKAP Commissioning Update Aidan Hotan

17 Layout of MRO Central Core BETA antennas circled in red. Central control building. Geothermal cooling field. 17 ASKAP Commissioning Update Aidan Hotan

18 Data Flow at the Telescope Information from 36 beams, 0.6 Tb/s PAF on Antenna RF over Fibre Digitiser Beamformer Information from all 188 PAF elements, 2 Tb/s (once digitised) x36 Antennas Inside central MRO building FPGAs Correlator PAF on Antenna RF over Fibre Digitiser Beamformer Radio Astronomy School Introduction to ASKAP Aidan Hotan

19 Digitisers and Beamformers Radio Astronomy School Introduction to ASKAP Aidan Hotan

20 Data Flow out of the Correlator Data rate out of the correlator is roughly 2.5 Gb/s, requiring a dedicated high-speed optical fibre link to a supercomputer. Imaging occurs via an automated pipeline. Raw visibilities are too large to archive, so processing needs to be right first time! Correlator has to be onsite. Data rates in are too high for economical longdistance transmission. Off-site, at the Pawsey Centre in Perth Astronomer Astronomer Correlator Visibilities Supercomputer (Imaging & Calibration) Images Archive (Storage & Interface) Astronomer Astronomer Radio Astronomy School Introduction to ASKAP Aidan Hotan

21 Commissioning Results from BETA Six antennas with firstgeneration PAFs and electronics. Total bandwidth 304 MHz. 1 MHz or 18.5 khz spectral resolution (up to 16,416 spectral channels). 9 dual-pol beams available. Degraded Tsys above 1 GHz. Commissioning team of 14 astronomers and engineers Radio Astronomy School Introduction to ASKAP Aidan Hotan

22 Observing Large Areas of the Sky (50 square degrees) 5.5 degrees Image made by Ian Heywood Radio Astronomy School Introduction to ASKAP Aidan Hotan

23 Detecting HI Absorption Radio Astronomy School Introduction to ASKAP Aidan Hotan Image made by James Allison

24 Widefield Spectral Line Observations Image made by Paolo Serra Radio Astronomy School Introduction to ASKAP Aidan Hotan

25 Multiple Objects in One Field Image made by Ian Heywood Radio Astronomy School Introduction to ASKAP Aidan Hotan

26 Testing Beam Polarisation Characteristics I Q U V Image made by Bob Sault 26 ASKAP Commissioning Update Aidan Hotan

27 Conclusions BETA is already dong science! The flexibility of adaptive beamforming is a huge advantage. We are learning many technical lessons to do with beamforming, PAF polarisation characteristics and data processing strategies. See for details of the BETA system. Contact Dave McConnell if you are interested in working with us on BETA. ASKAP will add rapid survey capabilities to the existing world-class radio astronomy facilities in Australia. Survey science data should be reaching the astronomy community in the next year or two (PhD / post-doc opportunities!) Radio Astronomy School Introduction to ASKAP Aidan Hotan

28 Thank you CSIRO Astronomy and Space Science Aidan Hotan ASKAP Deputy Project Scientist t e aidan.hotan@csiro.au w CSIRO ASTRONOMY AND SPACE SCIENCE